Toughness and Friction of Model Polyelectrolyte Gels

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NON-TECHNICAL SUMMARY:Polymer gels are flexible, solid materials containing a high proportion of a small-molecule component like water. The aim of the project is to develop polymer gels that are very strong, and that can also be used to produce exceptionally slippery surfaces with ultra-low friction. The possibilities have been illustrated by nature, in the form of certain types of cartilage tissue that are responsible for lubricating load-bearing joints in the body. The availability of synthetic versions of these tissues would transform orthopedic practice, extending the life of hip and knee replacements or even making these replacement procedures unnecessary. In addition, materials with the sought after combination of properties can be used as contact surfaces in a variety of manufacturing settings, enhancing energy efficiency by reducing the energy lost due to frictional heating. Past efforts to produce high-strength, low-friction gels have been limited by the fact that the same factors that enhance strength generally increase the friction as well. Two elements of the project will be undertaken in pursuit of the overall project aim. The first of these is the synthesis and testing of new gels designed to break the connection between gel strength and gel friction. These gels have features in common with their natural counterparts (like cartilage tissue), but can be tuned by altering certain details of the gel structure. The second element of the project is more conceptual, and involves the development of a general design strategy to guide future synthesis of high-strength, low-friction gels. TECHNICAL SUMMARY:The goal of the project is to develop high-toughness gels with ultralow friction, and to develop a generalized design strategy for producing these types of materials. The work is based on the synthesis of hybrid gels consisting of a charged, polyelectrolyte network to which multivalent cations have been added. These materials can have an extraordinarily high mechanical toughness, resulting from energy dissipation mechanisms arising from the presence of both weak and strong bonds in the same material. Because the development of a highly lubricious, low-friction material requires that energy dissipation be minimized, it is particularly challenging to maintain material toughness while simultaneously reducing the friction. The working hypothesis for the project is that friction and toughness are controlled by energy dissipation mechanisms that are operative on different time scales. The project is designed to elucidate these effects in a set of model gel systems that enables the relevant time scales to be controlled. The most important time scales are the force-dependent lifetimes of the weaker bonds, and the rate at which these weak bonds are able to reform after they are broken. The experimental model systems are based on gels formed from the controlled assembly of high molecular weight triblock copolymers with a poly(methacrylic acid) (PMAA) midblock and poly(methyl methacrylate) (PMMA) end blocks. Assembly of these polymers results in the formation of polymer gels with the strong bonds consisting of glassy PMMA aggregates. The weak bonds are introduced by complexation of midblock carboxylates with atomic or polymeric cations, with bond formation times and bond lifetimes controlled by the specific cations that are chosen. The tunable and quantifiable nature of the weak and strong bonds in these systems will enable the development of widely applicable design principles for producing high-toughness materials with the desired frictional properties.